Longest time ever spent on a puzzle...

I thought I would ask what the longest time anyone has ever spent on an individual puzzle?

For me it was an 11 grid sudoku puzzle that I did once that was really hard and required lots of tricky solving rules.

Also the fact I made a mistake on it but didn't realise for some time I had made one meant it took even longer. I reckon that in total I was probably working on it for something like three days on and off!

However it was worth it in the end as it felt really good to actually finish it